Hey all, good news! I now have r.sun written to run in OpenCL in GRASS 7. My test shows a 20x speedup over the original code. ("r.sun numpartitions=3 elevin=gauss glob_rad=glob_rad_ocl day=180")
Feel free to download the code from here:
http://github.com/mailseth/OpenCL-integration-for-GRASS---GDAL
It's currently setup for compiling on my machine, a Mac. You'll need to play with the makefile to get it to work otherwise.
I've only done limited testing, so if you find something that makes it error, please send a helpful bug report my direction. You could also send me a data file and the exact commands needed to replicate the problem.
~Seth